1. Field of Invention
This invention relates to a thrust ball bearing unit, and more particularly to an improved thrust ball bearing unit for use in a suspension of an automobile.
2. Description of the Prior Art
Numerous types of thrust ball bearing units are known in the prior art for use in a suspension of an automobile and the like. For example, a thrust ball bearing unit of this type is known from DE-GM No. 26 58 748.4 and French Patent Application No. 77-39083.
Thrust ball bearing units are widely used in MacPherson strut type automobile suspensions, and most parts which are in direct contact with the thrust ball bearings are made of sheet metal so that dimensional tolerances with respect to flatness are low, and the thrust ball bearings are in cantilever contact with races, thus resulting in problems such as fretting corrosion due to minor slip as well as noise.
There is another problem in that muddy water disperses into a tire housing of the automobile and rusts the seal plate, and rust or dust thus intruding into the sliding channel may damage the ball bearings.
In order to overcome the conventional disadvantages, a plurality of evenly spaced metal thrust ball bearings are rotatably held into an integral synthetic resin casing. As shown in FIGS. 13-16, a thrust ball bearing 4 is rotatably held between an upper race 2 and a lower race 3 which are covered by an upper casing 5 and a lower casing 6.
An embodiment of the MacPherson strut type automobile suspension is shown in FIG. 16, wherein a thrust ball bearing unit 10 including a pair of thrust ball bearings 1 is integrally enclosed in a synthetic resin casing, and more particularly an upper mount 34 is secured to an inner wall of a body 33 by a mounting screw bolt 47 and a nut 47a, a rod end plate 32 is provided on the upper mount 34 by a rubber mount 35, a screwed end portion of a shock absorber rod 37 is inserted into an opening of the rod end plate 32 and a screw nut 38 is screwed on the screwed end portion of the rod 37.
The upper casing 5 is inserted into a cylindrical portion 45 of the upper mount 34 which serves as a bearing holder as well. A cylindrical portion 41 of a spring seat 39 is inserted into an inner periphery of a lower casing 6 of the thrust ball bearing, an end portion of a coil spring 42 is inserted into an elastic seat 40, and another end of the coil spring 42 is held by a spring seat 44.
High dimensional tolerances related to leveling are required for the members enclosing the thrust ball bearings. Sheet metal is usually push-fitted into the upper mount 34 of the suspension so that a dimensional tolerance threat is low, and stress is imparted to the sliding channels of the inner and outer races which causes a longitudinal scar to be formed along the sliding channels.
To this end, there has been proposed another thrust ball bearing unit having its enclosure previously processed to be flat and level and mounted on the upper mount 34 or on the spring seat 44. But a head of the bolt 47 securing the upper mount 34 to the body 33 projects outwardly so that the thickness of the upper casing 5 becomes uneven so as to cause cambering during processing.
In order to overcome this disadvantage, a thrust ball bearing shown in FIG. 14 has been proposed, wherein an annular groove 49 is formed in an upper casing 5, a number of ribs 7 are radially provided in the annular groove 49 with the center of the bearing as their centers, and an upper race 2 having a number of equally spaced ball bearings 4 is brought into contact with these ribs 7, but the ribs are not formed at a portion through which a head of the fastening bolt 47 is located.
In the thrust ball bearing unit shown in FIGS. 13-15, an upper face which is in contact with a lower face of the upper casing 5 is flat so that ill-balanced impact is imparted to the thrust ball bearing 4 due to a rough seat of the upper mount 34, although the upper casing 5 is made of synthetic resin. This is presumably the reason why a number of ribs 7 are provided at equal spaces between an outer peripheral wall 5a and an inner peripheral wall 5b of the upper casing 5 so as to increase stiffness of the bearing unit, and even if the upper and lower races 1 and 2 are even, the upper casing 5 is not distorted appreciably in accordance with the roughness of the upper mount 34.
When such a thrust ball bearing unit having a synthetic resin casing which includes thrust ball bearings is used for a load splitting type suspension mounted directly on the body 33, the upper and lower races 2 and 3 are distorted which causes rolling vibrations and which causes minor rolling vibrations to be transmitted to the body 33.